This is a Court term. Most monarchs had posts filled in their courts by ordinary gentlemen, who came from the lower nobility or were commoners. In Great Britain the Queen still has Gentlemen at Arms, Gentlemen in Waiting and so forth.
It is my belief that the term "Gunmaker in Ordinary" implied that the individual granted the title was the royally appointed gunmaker to the King. It was perhaps the precursor to the granting of warrants to gunmakers, a fair few of which the Prince of Wales dished out in the late 1800s and early 1900s.
Tim
